The Rotation / Vibration Spectrum of HCl

INTRODUCTION

A diatomic molecule differs from an atom in having additional degrees of freedom caused by the ability of the molecule to rotate and vibrate about its center of mass. These motions can be, in first approximation, considered separately and lead to sets of energy levels, , but with relatively small values of . Since = h, the frequencies of electromagnetic radiation absorbed or emitted by a transition between two of these levels will lie in the infrared (frequency (nu) > 10¹³Hz, wavelength (lambda) < 20 m) or microwave regions ( < 10¹¹Hz, lambda > 20m) of the electromagnetic spectrum.

The simplest models for the motion of a diatomic molecule in its electronic ground state are the harmonic oscillator which describes the vibrations of the two atoms along the bond direction and the rigid rotor which approximates the rotation of the molecule in space. Since a diatomic molecule is under investigation, only one bond can vibrate and the rotation of the molecule occurs only about an axis perpendicular to the bond direction and through the center of mass.